The present invention relates to tunable optical filters having retained complementary outputs.
Optical fiber wavelength division multiplexed (WDM) communications systems are theoretically capable of extremely high data rates (terabits per second), meaning that many channels of gigabit rate data can theoretically be carried on a fiber, via wavelength division multiplexing.
Currently the two methods of constructing nodes on a fiber network are Optical to Electronic to Optical (OEO) conversion and fixed optical add/drop filters. OEO is the most common method, but is very expensive. The signals which are not being dropped are used to modulate lasers and the resulting wavelengths are multiplexed back in. Much of the hardware is data rate dependant.
Fixed optical add/drop nodes are simpler and less expensive, but must be replaced when any change is made. In addition, certain paths through the network are blocked from use, as no single wavelength can connect them, If the network is manually configured to remove a given block, another blocked path is inevitably created. This problem grows rapidly with increasing network complexity.
The utility of fiber optic systems has been limited because a truly useful optical tunable add/drop filter requires four characteristics:
1) Flat-topped pass bands, so that the modulation sidebands of the signal (where all of the information resides) are not attenuated;
2) Accurate tunability;
3) Retaining of the complementary output so the filter can add and drop; and
4) A switchable null/all-pass state, such that a filter can be switched into the all pass state, tuned to a different wavelength, and switched back to add/drop mode, all without interrupting, even momentarily, the continuity of other wavelengths passing through the filter and switch.
Currently, optical add/drop filters lacking at least one of the above characteristics are used to extract desired frequencies. FIG. 1 (prior art) shows a conventional fixed-wavelength optical add/drop filter system 100, based on a thin-film interference filter 108. FIG. 2 (prior art) shows the frequencies transmitted and reflected by filter 108 of FIG. 1.
Interference filters are a relatively inexpensive, mature technology. It is common to get flat-topped pass-bands and channel spacings down to 100 GHz (0.8 nm, in the 1550 nm communications band). Interestingly, interference filters can theoretically be tuned across a significant bandwidth by changing the angle of incidence of the light striking the filter. The limitation preventing the interference filter system 100 from being both tuned (rotated) and used as add/drop filter is the great difficulty of tracking the reflected output 106 (at Port B) as the filter is rotated. As a result, no tunable interference filters offer complimentary outputs 106xe2x80x94only the output 104 that passes through filter 108 is kept and the rest of the light is dissipated. Thus, despite the advantageous features of interference filters (low cost, flat passband shape, ready availability) commercial tunable add/drop filters have not yet been based on this technology.
Thus, filters with appropriately flat-topped pass bands (such as thin film interference filters) either are not tunable or, if tunable, do not have complementary outputs. Other tunable filter technologies, such as acousto-optic filters and Fabre-Perot filters cannot be constructed with flat-topped pass bands. No prior art filter design has satisfactorily dealt with the requirement for a switchable null/all-pass state for noninterference.
A need remains in the art for tunable drop filters and add/drop filters which retain the complementary output and have flat topped pass bands for use in WDM communication systems.
An object of the invention is to provide tunable drop filters and add/drop filters which retain the complementary output and have flat topped pass bands for use in WDM communication systems.
A tunable drop filter system according to the present invention includes some sort of tunable filter (thin film biregringent, holographic Bragg grating, beamsplitter, interference thin film) to divide the input beam into a dropped beam and a passed beam. A mirror is placed adjacent to the filter, such that their extended planes have a dihedral angle of less than 180xc2x0, and the input beam is directed at the filter such that the portion of the beam reflecting off the filter also reflects off the mirror. The passed beam, then, reflects off of the filter and the mirror and is directed to a fixed location, and the dropped beam passes through the filter. The filter is tuned by rotating the filter/mirror assembly around an axis formed where their extended planes meet.
For example, a tunable add/drop filter system for dividing an input beam into a dropped beam and a passed beam could comprise an interference filter and a mirror placed at an angle under 180xc2x0 to the filter to form a reflector assembly. The reflector assembly is rotatable about the vertex of the assembly angle, to tune the filter.
The input beam is directed such that the portion of the input beam that reflects off of the interference filter reflects off of the mirror. In this filter system, the dropped output passes through the filter, and the passed output reflects off of the filter and the mirror at a fixed angle from the input beam. The passed output beam is translated sideways from the input beam by a fixed amount regardless of the rotation of the reflector assembly.